1. Field of the Invention
The present invention relates to printers, copiers, facsimile machines, and other electrophotographic image forming apparatuses. More particularly, the present invention relates to a wet-type developing device and a wet-type image forming apparatus that employ wet-type development as a developing method.
2. Description of the Related Art
Various wet-type image forming apparatuses have been proposed, and such a conventional wet-type image forming apparatus employs wet-type electrophotography that allows high-quality image output using toner with a small diameter, as compared with dry-type electrophotography. Japanese Laid-Open Patent Publication No. 2012-128094, Japanese Laid-Open Patent Publication No. 2012-068372, and Japanese Laid-Open Patent Publication No. 2011-197216 disclose wet-type developing devices and wet-type image forming apparatuses that employ wet-type electrophotography.
In a case of developing charged toner particles on a developing roller, and then cleaning the charged toner from the developing roller using a cleaning blade, the cleaning process tends to end in failure due to high adhesion of the toner particles to the developing roller.
In order to facilitate the cleaning process, a neutralizing charger is disposed on the upstream side of the cleaning blade. The neutralizing charger applies electric charge which is opposite in polarity to the electric charge applied to the toner particles, to the toner on the developing roller, thereby lowering the charged level of the toner.
The electric charge applied to the toner particles by a static charger decays with a lapse of time. Therefore, flow-in current from the static charger and flow-in current from the neutralizing charger are appropriately set such that the flow-in current from the static charger becomes larger in absolute value than the flow-in current from the neutralizing charger.
A corotron charger has a polarity-dependent discharge characteristic (an amount of discharged current relative to applied voltage to a wire), and tends to be discharged in a case of negative polarity rather than positive polarity. As a result, an absolute value of flow-in current into a developing roller relative to an absolute value of applied voltage to a wire (a V-I characteristic) is large in the case of negative polarity rather than positive polarity.
In a case of using a photoconductor made of a-Si, preferably, toner particles are positively charged. Typically, a static charger (positive discharge) required to be applied with high voltage has a low V-I characteristic whereas a neutralizing charger (negative discharge) required to be applied with low voltage has a high V-I characteristic.
Applied voltage to a wire of a charger has an upper limit determined from a critical limit of leakage initiation, and a lower limit determined from discharge initiation voltage. Negative polarity causes a region where discharge is unstable even after the start of discharge, which results in uneven discharge. Therefore, the applied voltage to the wire is set to be higher than the discharge initiation voltage in order that the charger is used in a region where the discharge is stable. The uneven discharge refers to such a phenomenon that a charger has a discharge stable area and a discharge unstable area depending on a location of a wire.
Leakage from a static charger causes uneven static charge on a toner layer, and the uneven static charge appears as noise on an image. On the other hand, uneven discharge by a neutralizing charger causes failed toner neutralization depending on a place. Consequently, cleaning ends in failure at the place, and the failed cleaning appears as a streak.
Applied voltage to a wire has an upper limit and a lower limit as described above. When a static charger and a neutralizing charger are corotron chargers which are similar in sectional configuration to each other as before, applied voltage to a wire of the static charger tends to exceed the upper limit (critical limit of leakage initiation) because the static charger has a large amount of required electric charge and a low discharge characteristic. On the other hand, applied voltage to a wire of the neutralizing charger tends to fall short of the lower limit (critical limit of negative uneven discharge) because the neutralizing charger has a small amount of required electric charge and a high discharge characteristic. Moreover, the upper and lower limits of the applied voltage restrict a usable range of flow-in current.
The static charger and the neutralizing charger may be used such that flow-in current into a developing roller falls within the range set by the upper and lower limits. However, the required flow-in current is determined from, for example, a charged level of toner, an amount of toner (based on a kind of a sheet of paper), and an external environment. In actual fact, therefore, an adjustable width of the flow-in current is required to be widely secured. Desirably, the adjustable width can be widely secured as much as possible.
A toner dispersing member (such as an AC roller) may be disposed on the upstream side of a cleaning member in order to improve cleaning performance. In such a case, however, if a neutralizing charger is unevenly discharged because applied voltage to a wire is low, cleaning ends in failure at part of a cleaning portion. Further, a developer is deposited on part of the cleaning portion.